The present invention relates to an image density reading apparatus which is used for an image forming apparatus such as a copying machine.
A copying machine commonly has the function of obtaining an image with the most adequate density on a transfer paper by adjusting the bias of a developing sleeve automatically according to the density of a document.
The above-described automatic adjusting operation can be conducted as follows. The image density signal obtained by a sensor during EE scanning is amplified and inputted into CPU (Central Processing Unit), wherein EE scanning is the motion of a light source which is moved along a document in order to obtain the density information of the document. An EE histogram is made according to the density information processed by CPU so that the density distribution of the document can be obtained. Then, the most adequate developing sleeve bias can be determined according to the density histogram.
Accordingly, the correct processing of the EE signal is essential in order to obtain the adequate developing sleeve bias. For that reason, copier manufacturers adjust all their products in their manufacturing lines as follows. Fine adjustment is made to the gain and offset in the EE signal processing circuit in order to avoid the variation of read-out density caused by the inaccuracy of the sensor and the error of the resistance of resistors composing the circuit so that the same density data can be obtained by every copier when it conducts density reading of the same document.
FIG. 7 is a circuit diagram of a conventional image density reading apparatus.
In this conventional example, the EE signal obtained by the EE scanning is processed as follows. The light emitted by the exposure lamp 2 is reflected by the surface of the document 1, wherein the surface is to be copied. The reflected light is converted into an EE signal by the EE sensor (the light receptor element) which is provided beside the optical system 3. This EE signal is amplified by the gain selecting circuit 5 and the offset adjusting circuit 6 so that the voltage of the signal can be converted into the voltage which is able to be inputted into the analog terminal of CPU 7.
The gain selecting circuit 5 is composed of the operation amplifier 8, the gain adjusting resistor R1, and the resistors R2 to R7. The output of the EE sensor 4 is inputted into the reverse input terminal of the operation amplifier 8 through the variable resistor R1 and the resistor R2. The voltage into which the power source voltage (10 V) is divided by the resistors R3 and R4, is inputted into the non-reverse terminal. The resistor R5 is provided between the output terminal and the non-reverse input terminal of the operation amplifier 8. Furthermore, the resistors R6 and R7 divide the voltage of the output signal of the operation amplifier 8. Its separating point is connected to the M side of the switch SW 1. The gain of this gain selecting circuit appears as G=R5/(R1+R2), wherein each letter in the equation represents the resistance of each resistor. Consequently, the gain can be varied in the range of 2 to 10, for instance, by manually making fine adjustment to R1 for use in adjusting the gain.
The offset adjusting circuit 6 is composed of the operation amplifier 9, the variable resistor R10 for use in adjusting the offset, the resistors R8, R9, R11, R12, and R13. The output signal of the operation amplifier 8 is inputted into the reverse input terminal of the operation amplifier 9 through the resistor R8. The voltage into which the power source voltage (10 V) is divided by the resistor R9, the variable resistor R10, and the resistor R11, is inputted into the non-reverse input terminal of the operation amplifier 9. The resistor R12 is provided between the output terminal and the reverse input terminal of the operation amplifier 9. The resistor R13 is provided between the output terminal of the operation amplifier 9 and the ground. The variable resistor R10 for use in adjusting offset is manually adjusted.
When the apparatus is set to the actual operation mode, the switch SW1 is switched to the P side, wherein the actual operation mode is the mode in which the document density is actually read in. When the apparatus is set to the adjusting mode, the switch SW1 is switched to the M side.
A predetermined display is indicated by the monitor 10 according to the level of the inputted EE signal.
The bias circuit 12 supplies an adequate developing bias corresponding to the EE signal level according to the direction sent from CPU 7. The amount of toner 14 which adheres to the drum 13 is controlled by the developing bias.
The operations of the conventional example will be explained as follows.